minor #16871 Update http_kernel_httpkernelinterface.rst (shamsibabakhanov)

This PR was submitted for the 6.1 branch but it was merged into the 4.4 branch instead.

Discussion
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Update http_kernel_httpkernelinterface.rst

The esi tag is not shown in the documentation page.

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Commits
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172d652 Update http_kernel_httpkernelinterface.rst

Author: javiereguiluz

create_framework/http_kernel_httpkernelinterface.rst

@@ -155,7 +155,7 @@ rest of the content? Edge Side Includes (`ESI`_) to the rescue! Instead of
 generating the whole content in one go, ESI allows you to mark a region of a
 page as being the content of a sub-request call:
 
-.. code-block:: text
+.. code-block:: xml
 
     This is the content of your page